Perfusion measurements attempt to determine the quantity of blood moving through a capillary network of a volume of tissue. The ability to perform tissue perfusion measurements, on a continuous basis, with accuracy and safety is important to both the clinical and research branches of the medical community because of the fundamental role perfusion plays in physiological processes. Likewise, perfusion measurements are of value in assessing the state of diseased tissue. While existing techiniques for performing perfusion measurements fall into a number of categories, measurements are, generally, made by monitoring the movement of an indicator through the tissue's capillary network. In such systems the concentration of the indicator at a point of known distance from the source may be employed in a mathematical model of the indicator diffusion system to derive the perfusion of the tissue volume.
The existing measuring methodology can be divided into two categories, namely, those which provide steady-state values and those which provide continuous values. Steady-state techniques are those which require that the perfusion within the tissue volume to be examined remain unchanged during the time required to perform a measurement. Continuous techniques permit the observation and measurement of perfusion transients.
The steady-state category includes methods which permit only one measurement per subject, e.g., C.sup.14 -antipyrine autoradiographic methods and those which permit multiple measurements. These methods require minutes per measurement to be performed, e.g., inert gas clearance, H.sub.2 -short interval clearance, positron emission tomography, microsphere entrapment, and thermal clearance. Selection of a method from this category depends upon the size of the measurement volume for which the perfusion measurement is to be made, subject safety, measurement time required and equipment requirements, e.g., the autoradiographic method permits the smallest volume resolution, requires the longest measurement time (about two weeks per measurement) and that the subject is destroyed to perform the measurement.
Continuous techniques attempt to read transient changes in perfusion. This category includes the laser doppler method, the nuclear magnetic resonance method, video image analysis and heated probe methods. The first two mentioned techniques permit perfusion measurement non-invasively; however the measurement volumes are large. The video method is useful for surface capillary network perfusion measurements. The heated probe techniques measure perfusion from small volumes, cause minor tissue damage and require minimal instrumentation.
As can be seen from the above discussion, the above techniques suffer from one or more of a number of serious problems, including, inability to deal with small volumes, tissue damage, a requirement for extensive instrumentation, long measurement time and the like.